From: Steve Hansen Newsgroups: sci.electronics Subject: Vacuum Newsletter #3 Date: Tue, 28 Feb 95 19:12:59 -0500 ************************************************************************** * ----------------------------------- ----------------- * * the Bell Jar (electronic version) #3 (February 1995) * * ----------------------------------- ----------------- * * * * This newsletter contains material which has been extracted from the * * hard-copy newsletter of the same name. Devoted to the vacuum * * experimenter, the intent of "the Bell Jar" is to broaden interest * * in vacuum technology through useful discussions of theory and * * technique, and to present ways in which a variety of apparatus may * * be assembled using common and inexpensive materials. Information * * on "the Bell Jar" may be obtained by sending email to the editor, * * Steve Hansen, at shansen@tiac.net or by writing to 35 Windsor Dr., * * Amherst, NH 03031. Please feel free to circulate this electronic * * version, intact, to others who might be interested in the subject * * matter. New numbers will be mailed at approx. quarterly intervals. * * Email subscriptions are free and may be obtained by contacting the * * editor. Comments, contributions and criticisms are always welcome. * * Copyright 1995, Stephen P. Hansen. * ************************************************************************** In this issue: An update on the use of the 6BK4 beam tube as a cold cathode x-ray source. The Radiometer What's in the current issue of "the Bell Jar" (Print Version) "An Experimenter's Introduction to Vacuum Technology" *********************** GENERATING X-RAYS WITH RECEIVING TUBES - Addendum In #2 Bob Templeman described his experiments using the 6BK4 TV high voltage regulator tube as a cold cathode generator of x-rays. James O'Neal of Alexandria, VA has provided some additional background on this tube that may explain why some tubes work well and others don't. "Those of us who are old enough may remember that there was something of a flap in the color TV industry during the late 60's when it was discovered that there was x-ray leakage from certain models of color TV. This was attributed primarily to the (then) quintessential 6BK4. "The tube manufacturers solved the problem by redesigning the tube, going to a heavily leaded glass formulation. The "improved" 6BK4 is instantly distinguishable just by lifting it - it weighs substantially more than any "normal" tube of roughly the same dimensions. For experimenters, there may be some of the thin-skinned units either in old junked sets or in service shops that have been around for a while. A distributor of tubes such as Antique Electronic Supply (Tempe, AZ, 602-820-5411) might also be able to help. "I did some experimentation with the original 6BK4 at the time of the flap and I found that the tube, when driven hard enough, could indeed produce x-rays. (I still have a nice mental picture of the glass envelope of one of them brightly fluorescing a lime green color.)" *********************** THE RADIOMETER The first 3 paragraphs are drawn from material developed by Franklin B. Lee in "Experiments in High Vacuum" (Morris and Lee, Buffalo, NY, ca. 1960) and is used with permission. The complete text, including instructions on building a radiometer from scratch, is reprinted in "the Bell Jar" Volume 4 Number 1, Winter 1995. A radiometer consists of a set of vanes, each shiny on one side and blackened on the other, that is mounted in an evacuated vessel. When exposed to light, the vanes revolve. The first radiometer was constructed to settle the controversy regarding whether light exerts a force. The idea was that a reflecting surface would experience a greater force from the light than an absorbing one. The instrument was therefore made in the now familiar form. Unexpectedly, the opposite effect was observed. The blackened vane retreated from the light source. We now know that the black surface is warmer than the shiny one and that gas molecules will recoil faster from the hot surface. The slight difference in molecule recoil is what causes the device to spin. (Later experiments in a much better vacuum have confirmed that light does exert a very small pressure. The action of the radiometer depends upon striking a balance between molecular drag and recoil. At higher pressures, drag will dominate and the vanes will fail to spin. At lower pressures, there are too few recoiling molecules to drive the vanes. The optimum balance occurs at a pressure of about 60 mTorr (60 microns Hg). By using a suitable tachometer (e.g. a 'Strobotach' or an electronic counter with a photocell that detects the interruption of a light beam by the vanes) it would be possible to measure the change in rotational velocity with changing pressure, given a constant light input. While the radiometer is not a very good gauge in itself, Dushman in "The Scientific Foundations of Vacuum Technique" noted the use of the radiometer to determine when the vacuum in an incandescent lamp had reached the required level for sealing-off. At the proper pressure, the vanes would cease to rotate, even in very bright illumination (this would be on the low side of the ~60 mTorr peak). He also noted that the level of vacuum could be quantitatively determined by shaking the bulb to set the vanes in motion and then noting the rate at which the spinning ceases. This notion has been embodied in the modern spinning rotor (molecular drag) gauge. Ready-made radiometers are available from science supply houses. Also they are increasingly popular as window ornaments and can often be obtained for about $10 from local craft shops. I've also seen radiometers in the windows of "New Age" boutiques, leading me to wonder what strange powers people might attribute to them. For tinkerers, the disadvantage of ready-made radiometers is that they are sealed off. Fortunately, the glass pump-out tube is readily accessible. With a file, nick the end of the pump-out and break the tip off. (I'd suggest placing a piece of rubber or vinyl tubing over the glass to prevent cuts). Using epoxy cement, seal a length of 5/16" OD brass tubing (K&S Engineering, available at well stocked hobby and hardware stores) to the bulb. Be careful not to get epoxy into the original evacuation ports in the stem. The hanging style of bulb is the most convenient to use. If no local source can be found, these are available from Edmund Scientific Co., 101 E. Gloucester Pike, Barrington, NJ 08007-1380 as Catalog #38,510 at $10.95. *********************** WHAT'S IN "the BELL JAR" (Print Version) The first issue of Volume 4 was mailed in January. Articles included Geiger tube construction, a regulated 30 kV (max) power supply for vacuum experiments, conversion tips for 2 stage refrigeration service vacuum pumps (a low cost way to <20 mTorr at 3 to 4 cfm) and a Pirani gauge made from small light bulbs. Plans for the next issue (Spring) include some historical perspectives on amateur vacuum, practical & simple plasma sources using microwave oven components, a thermistor gauge developed at the University of Alberta and more useful tips and ideas. *********************** AN EXPERIMENTER'S INTRODUCTION TO VACUUM TECHNOLOGY This booklet, drawn from the first 3 years of "the Bell Jar", will be available in March from Lindsay Publications, PO Box 538, Bradley, IL 60915-0538. Topics covered include vacuum basics, low cost mechanical pumps, construction of two simple gauges (discharge tube and thermocouple), high vacuum compatible construction techniques with an emphasis on using common materials, and a series of projects that may be conducted with a good mechanical pump. There are ample references and a supplier list. 39 pages, 8-1/2 x 11 format.